Squid Microscope

ABSTRACT

In order to measure magnetic characteristics of a sample, the latter is positioned above the SQUID so that both the SQUID and the sample are supported in a vessel in a gaseous nitrogen space above a liquid nitrogen coolant so that the measurement can take place at ambient pressure and in a nitrogen atmosphere.

FIELD OF THE INVENTION

[0001] Our present invention relates to a SQUID microscope for measuringor detecting magnetic characteristics of a sample and, more particularlyto a method of and to an apparatus for determining magnetic propertiesof a sample utilizing a SQUID (superconductive quantum interferencedevice).

BACKGROUND OF THE INVENTION

[0002] T. J. Shaw et al, “High-T_(c) SQUID Microscope Study of theEffects of Microstructure and Deformation on the Remanent Magnetizationof Steel”, IEEE Transactions on Applied Superconductivity, 1999 (9)2,pages 4107 to 4110 describe a SQUID microscope for determining magneticcharacteristics of a sample. The SQUID is here mounted on a movable rodand the rod and SQUID are provided in a vacuum chamber which is coupledwith a liquid nitrogen supply unit. A sample holder carrying the sampleis disposed outside the vacuum chamber and a sapphire window separatesthe sample and the SQUID.

[0003] A drawback of this system is that the sample cannot be placed asclose to the SQUID as might be desired. Also, in order to effect themeasurement, a vacuum must be generated in the vacuum chamber.

[0004] From L. N. Vu et al, “Design and Implementation of a ScanningSQUID Microscope”, IEEE Transactions on Applied Superconductivity,Volume 3(1), 1993, pages 1918 to 1921, a SQUID microscope is known. Thismicroscope encompasses a chamber with liquid helium. Connected to thischamber is a sample chamber insulated by an intermediate vacuum layer.In the sample chamber, both the sample and the SQUID are provided, theSQUID being mounted on a movable rod. The rod can be inserted into andwithdrawn from the sample chamber and can be sealed toward the exteriorvia a metallic bellows. The sample and SQUID are cooled by gaseoushelium which is in a thermal connection with a liquid helium supplychamber. To cool the microscope, an outer cylinder cooled by liquidnitrogen is additionally provided. The insertion and sealing of thesample in this SQUID microscope is expensive and the need forrecirculating gaseous helium is as a rule complex and costly.

OBJECTS OF THE INVENTION

[0005] It is the principal object of the present invention to provide anapparatus which is constituted as a SQUID microscope and which bycomparison with earlier SQUID microscopes is simple and inexpensive andnevertheless can serve effectively for expelling magnetic properties ofa sample.

[0006] It is also an object of this invention to provide a method ofdetermining magnetic properties of a sample using a SQUID, wherebydrawbacks of prior systems are avoided.

[0007] Still another object of the invention is to provide a SQUIDmicroscope and a method of operating same whereby the spacing betweenthe SQUID and the sample can be optionally small.

[0008] A further object of the invention is to provide a method of andan apparatus for determining magnetic properties of a sample whichenables magnetic property measurement to be carried out at ambientpressure, i.e. without the need to evacuate the space containing theSQUID or sample.

SUMMARY OF THE INVENTION

[0009] These objects and others which will become apparent hereinafterare contained, in accordance with the invention in a method of measuringmagnetic characteristics of a sample which comprises the steps of:

[0010] (a) cooling a SQUID to a superconducting temperature enabling theSQUID to respond to magnetic characteristics of a sample;

[0011] (b) juxtaposing the SQUID at superconducting temperature and thesample in a gaseous nitrogen atmosphere at ambient pressure; and

[0012] (c) effecting a measurement with the SQUID at superconductingtemperature of a magnetic property of the sample in the gaseous nitrogenatmosphere at the ambient pressure.

[0013] The apparatus of the invention is thus a SQUID microscope whichcan comprise:

[0014] an upwardly open vessel;

[0015] a SQUID holder in the vessel configured to cool a SQUID to asuperconducting temperature enabling the SQUID to respond to magneticcharacteristics of a sample;

[0016] a SQUID mounted on the SQUID holder in the vessel and adapted tobe cooled to the superconducting temperature enabling the SQUID torespond to magnetic characteristics of the sample; and

[0017] a sample holder in the vessel carrying a sample juxtaposed withthe SQUID in a gaseous nitrogen atmosphere at ambient pressure wherebymeasurement of a magnetic property of the sample can be effected withthe SQUID at superconducting temperature in the gaseous nitrogenatmosphere at the ambient pressure.

[0018] The SQUID is preferably cooled in step (a) with liquid isnitrogen and the SQUID and sample can thus be provided in an upwardlyopen vessel containing this liquid nitrogen to a level just below theSQUID so that the SQUID and the sample are both disposed in the gaseousnitrogen atmosphere overlying the liquid level in this vessel. Thevessel itself may be a cryostat and the cryostat can be provided with alining of a μ metal shield.

[0019] The sample and hence the sample holder preferably is locatedabove the SQUID and the SQUID holder can be a sapphire rod which isimmersed in the liquid nitrogen.

[0020] More particularly, the method of the invention measures magneticproperties of a sample with a SQUID microscope in which the sample andthe SQUID are both in a protective gas atmosphere of nitrogen at ambientpressure, i.e. the pressure of the atmosphere surrounding the apparatusand normally approximately atmospheric pressure. The protective gasatmosphere prevents or limits the condensing out or the freezing out,(sublimation) of undesired components on the surface of the sampleand/or of the SQUID. Since the method operates at ambient pressure,expensive vacuum technology can be avoided. The sample change as a ruleis simple and quick. The protective gas atmosphere can be supplieddirectly by the coolant both therebelow, the liquid nitrogen evaporatingform its surface located preferably just below the SQUID.

[0021] The cooling of the SQUID with liquid nitrogen in the bathsurrounding the rod on which the SQUID is mounted, greatly simplifiesboth the generation of the protective gas atmosphere from the coolantand the cooling action.

[0022] The use of an upwardly open vessel can communicate with theambient atmosphere through the opening at the top of this vessel andwhich contains the liquid nitrogen amounts to a further simplificationof the apparatus since it can ensure the ambient pressure for the SQUIDand sample.

[0023] When the sample is located above the SQUID it can be raised orlowered and brought as close as may be desirable to the SQUID.

[0024] The vessel itself can be a Dewar flask or beaker composed ofglass, stainless steel or other metals commonly in use withlow-temperature systems and GFK.

[0025] The μ metal shield around the SQUID microscope of the inventionreduces or eliminates distortion of the magnetic field applied to theSQUID and thus improves the measurement precision. The shield can formthe opening communicating between the external atmosphere and thenitrogen blank in the vessel and this opening can be such as to justpermit passage of the sample holder.

[0026] When the sapphire rod is used as a support for the SQUID, it cansimultaneously serve as a heat transfer element between the liquidnitrogen bath and the SQUID. A sapphire rod has good thermalconductivity and very poor electrical conductivity.

[0027] The SQUID cooling can be independent from that of the sample andvice versa so that measurements can be taken even when the SQUID and thesample are at different temperatures.

[0028] When the gaseous nitrogen develops from the liquid nitrogen bathbelow the SQUID, it can serve to drive any moisture or air from thevessel and thus permit operation of the apparatus for long periods oftime. The fact that moisture is driven out of the system prevents theSQUID, which is located above the liquid nitrogen level in the nitrogenatmosphere from icing up. The evaporated coolant thus forms theprotective gas atmosphere.

[0029] Since the SQUID and the sample are not separated by a window inthe SQUID microscope according to the invention, the sample can bepractically brought into contact with the SQUID. A pressure sensor orcontact sensor can be provided to output a signal upon contact of thesample with the SQUID and thus allow a process adjustment of the spacingbetween the SQUID and the sample.

[0030] The SQUID microscope of the invention has not only the advantagethat it permits investigation of magnetic properties of the samples atambient pressure, but also that expensive vacuum pumps and liketechnology can be avoided. Condensation or other deposition of undesiredmaterials on the sample surfaces and/or on the SQUID are precluded andthe freezing out of water on the sample surface is especially avoided.

[0031] Access to the SQUID or the sample for replacement or checking andeven during the measurement (e.g. quality control) is ensured and theapparatus of the invention is especially characterized by permittingquick sample and SQUID replacement.

[0032] The distance between the sample and the SQUID can be as small as1 μm and, when the SQUID is a HTSL (high temperature superconductor)SQUID, the cooling is relatively simple and the SQUID and sample can beindependently cooled or heated.

BRIEF DESCRIPTION OF THE DRAWING

[0033] The sole FIGURE of the drawing is a schematic illustration of aSQUID microscope according to the invention.

SPECIFIC DESCRIPTION

[0034] The drawing shows schematically a SQUID microscope in which theSQUID 1 is mounted on a sapphire rod 2 which is mostly immersed in aliquid nitrogen bath 3. Because of its high thermal conductivity and itslow electrical conductivity, the heat is conducted away from the SQUID 1so as to maintain it at a temperature of, say, 77 K and can providesufficient cooling. The SQUID may be a high-temperature superconductorSQUID. The SQUID itself is located above the liquid/gas interface 10 ofthe nitrogen bath 10.

[0035] Because of evaporation of the liquid nitrogen from the bath,there is a gaseous nitrogen atmosphere 4 above the liquid nitrogen 3.This gaseous nitrogen displaces the air and any moisture from the regionaround the SQUID so that the SQUID 1 which remains for long periods oftime above the liquid nitrogen surface 10 without icing up.

[0036] The vessel for the nitrogen can be a simple cryostat 5, forexample a glass cryostat. The SQUID and its rod 2 are mounted in thevessel in the holder 6.

[0037] The sample 8, whose magnetic properties are to be measured, ismounted on a holder 7 which can form part of an x, y, z drive 11 capableof displacing the sample along the x, y and z axes to effect athree-dimensional scanning of the magnetic field around the sample 8 bythe SQUID 1.

[0038] The holder 7 can be equipped with a measure sensor or contactsensor 12 which can detect contact of the sample 8 with the SQUID 1 andthus, via the control for the drive 11, position the sample 8 at aprecise distance above the SQUID 1.

[0039] The vessel is provided with a μ metal shield 9 which preventsmagnetic distortion of the field detected by the SQUID. This shield canhave a small opening 13 around the holder 7 so that there is onlysufficient clearance for movement of this holder and enough space toenable the gas blanket 4 above the liquid nitrogen to communicate withthe ambient atmosphere. The μ metal shield 9 can thus provide optimumshielding of the SQUID 1 juxtaposed with the sample 8.

[0040] The measurement of the magnetic field of the sample may beeffected by the method described in either of the aforementionedreferences.

We claim:
 1. A method of measuring magnetic characteristics of a sample,comprising the steps of: (a) cooling a SQUID to a superconductingtemperature enabling said SQUID to respond to magnetic characteristicsof a sample; (b) juxtaposing said SQUID at superconducting temperatureand the sample in a gaseous nitrogen atmosphere at ambient pressure; and(c) effecting a measurement with said SQUID at superconductingtemperature of a magnetic property of said sample in said gaseousnitrogen atmosphere at said ambient pressure.
 2. The method defined inclaim 1 which comprises cooling said SQUID in step (a) with liquidnitrogen.
 3. The method defined in claim 2 wherein said SQUID and saidsample are provided in an upwardly open vessel containing said liquidnitrogen.
 4. The method defined in claim 1 wherein said SQUID and saidsample are provided in an a gaseous nitrogen atmosphere above liquidnitrogen.
 5. The method defined in claim 1 wherein said sample isdisposed above said SQUID in steps (b) and (c).
 6. A SQUID microscopefor measuring magnetic characteristics of a sample, comprising: anupwardly open vessel; a SQUID holder in said vessel configured to cool aSQUID to a superconducting temperature enabling said SQUID to respond tomagnetic characteristics of a sample; a SQUID mounted on said SQUIDholder in said vessel and adapted to be cooled to said superconductingtemperature enabling said SQUID to respond to magnetic characteristicsof said sample; and a sample holder in said vessel carrying a samplejuxtaposed with said SQUID in a gaseous nitrogen atmosphere at ambientpressure whereby measurement of a magnetic property of said sample canbe effected with said SQUID at superconducting temperature in saidgaseous nitrogen atmosphere at said ambient pressure.
 7. The SQUIDmicroscope defined in claim 6 wherein said sample holder is locatedabove said SQUID.
 8. The SQUID microscope defined in claim 7 whereinsaid vessel contains a cooling medium.
 9. The SQUID microscope definedin claim 8 wherein said cooling medium is liquid nitrogen.
 10. The SQUIDmicroscope defined in claim 9 wherein said vessel contains a bath ofliquid nitrogen below the gaseous nitrogen.
 11. The SQUID microscopedefined in claim 10 wherein said liquid nitrogen has a level below saidSQUID.
 12. The SQUID microscope defined in claim 6, further comprising ashield of μ-metal surrounding said SQUID and said sample.
 13. The SQUIDmicroscope defined in claim 6 wherein said SQUID holder is a sapphirerod.
 14. The SQUID microscope defined in claim 6, further comprising atemperature controller for said sample independent of aq temperature ofsaid SQUID.